This might work and is not a bad idea, but speaking from a little direct experience - be very careful. The file, especially a good one, will cut. If you are very careful to hold it flat, you might get away with it. The corner of the file, connecting with an inside radius of the belly of the knife, will dig in. The result can look like someone tried to strip a Busse at home or something
.
My approach, again with just having tried this once or twice, is to trace the outline with a
very sharp pencil (sharpie will bleed/wick and blurr the actual line). Do ALL of the sanding and shaping with the liners OFF of the knife.
In the process you trace the line, grind down to the line, check the fit. And repeat about 5-6 times until the fit is perfect. Here is another tip - micarta will leave a fuzzy burr after sanding that makes tracing the line the next time difficult. So lay the liner flat on a flattening plate and sand off the burr around the edge. Trace, sand, check, repeat.
Always trace the tang of the knife. It seems to work better than tracing the scales.
The holes in the tang make a perfect drill template. 1/4" drill bit. Drill one hole and use a rolled up piece of sandpaper to open up the hole (the fastener will not go in a drilled 1/4" hole in micarta with out opening it up). Drill one hole. sand. use a fastener as a dowel pin, then clamp the whole shebang down and drill the second hole. Sand, fit a second fastener, clamp, and drill the 3rd hole.
If it would help, I could do a tutorial someday with pictures.
Again, just done this myself once, so I could be wrong, too...